LTE (Long Term Evolution) is the next step in cellular Third-Generation (3G) systems, which represents basically an evolution of previous mobile communications standards such as Universal Mobile Telecommunication System (UMTS) and Global System for Mobile Communications (GSM). It is a Third Generation Partnership Project (3GPP) standard that provides throughputs up to 50 Mbps in uplink and up to 100 Mbps in downlink. It uses scalable bandwidth from 1.4 to 20 MHz in order to suit the needs of network operators that have different bandwidth allocations. LTE is also expected to improve spectral efficiency in networks, allowing carriers to provide more data and voice services over a given bandwidth. In order to do that, LTE uses Orthogonal Frequency-Division Multiple Access (OFDMA) which is a proven access technique, based on Orthogonal Frequency-Division Multiplexing (OFDM), for efficient user and data multiplexing in the frequency domain. Other wireless standards like WiFi (IEEE 802.11) or WiMAX (IEEE 802.16) also employ OFDM techniques.
One of the advantages of OFDM is its ability to resolve the frequency components of the received signal. Frequency resolution allows the receiver to determine the received signal to interference and noise ratio (SINR) corresponding to the different frequencies of interest or subcarriers. This set of SINR values is exploited by the receiver to derive the most suitable modulation and coding format to use when link adaptation is employed in the system. The receiver can obtain such modulation and coding format, and report it to the transmitter in order to optimize the transmissions for the most suitable operating point.
On the other hand, error detection and correction in received blocks (for example in the received packets) are long-standing techniques that have achieved stunning progress in the last decades. Error detection codes have the ability to detect that an error has occurred in a packet with high reliability, at the cost of some overhead usually appended at the end of the packet. Forward Error Correction (FEC) techniques encoding/decoding techniques are able to correct bit errors up to a certain limit depending on the channel and the characteristics of the underlying FEC code. While the FEC encoding process at the transmitter is computationally quite simple, FEC decoding at the receiver can be extremely time-consuming, for example, for state-of-the-art FEC techniques like turbo coding or Low-Density Parity Check (LDPC) coding. This computational burden introduces significant penalty in the overall latency of the system, which in turn constrains the maximum length of the transport network links, especially in so-called centralized or Cloud radio access networks (CRAN), between remote radio heads and the baseband unit. Said transport network, commonly known as fronthaul, is mainly constrained by the Hybrid Automatic Repeat Request Round-Trip-Time (HARQ RTT), which in LTE is equal to 8 ms. A significant proportion of the time budget for reception is spent on the FEC decoding process, thus imposing a maximum length for the fronthaul links.
Detecting (or predicting) errors in a received block prior to actually decoding it can be very advantageous for significantly reducing the overall latency at the radio access network. Early detection of packet errors can trigger retransmissions without having to perform complete FEC decoding of the packet. Moreover, in fully or partly centralized RAN architectures where FEC decoding is centralized, early triggering of retransmissions at the remote radio head can relax the time budget constraints for the fronthaul network by taking the FEC decoding process out of the critical HARQ loop.
There are partial solutions for time budget relaxation at the fronthaul network involving, for example, splitting the Medium Access Control (MAC) layer for early triggering of HARQ retransmissions. However these solutions do not perform early detection of packet errors, and instead both acknowledgments (ACK) and non-acknowledgments (NACK) are pre-processed at the remote radio head. This imposes limitations in terms of lack of flexibility to allocate different resources for a transmission and a retransmission.
More adequate solutions are therefore required in order to predict whether an error has occurred in a given received packet with sufficient reliability, without having to perform full FEC decoding of the packet (which involves significant penalty in terms of the overall latency of the system).